This invention relates to a method and a machine for producing bags containing fiber material, preferably loose tobacco.
This specification refers, without limiting the scope of the invention, to a machine for forming portions of loose tobacco, preferably of the type known as rolling tobacco, and filling it into bags.
The production of bags containing tobacco, used by pipe smokers or cigarette smokers who make their own cigarettes by rolling the tobacco in a piece of gummed paper, is usually implemented on machines of the type described, for example, in patent EP1454852B1, in which the portions of tobacco are placed in the bags without performing truly effective constant checks on quantity and position.
In effect, machines for producing bags containing tobacco are known which comprise metering units capable of separating and weighing single tobacco portions to be placed in bags.
These machines extend substantially vertically from a top section where there is a unit for feeding loose tobacco, to a base section where a collecting unit receives and conveys the single portions to a packaging station downstream.
More precisely, prior art machines comprise a unit for forming a continuous strand of loose tobacco extending along vertical duct from a hopper, which contains the tobacco, to a cutting device.
The device which cuts the continuous strand of tobacco is located at an outfeed section of the duct and divides the continuous strand into a succession of single tobacco portions.
Under the outfeed section, there is a primary drum having three angularly equispaced receptacles each designed to be aligned in turn with the outfeed mouth as the primary drum rotates about its axis of rotation.
Thus, once the cutting device has formed a single portion, that portion falls by gravity into the receptacle below it.
Also associated with the primary drum is a scale for measuring the weight of the tobacco portion after it has been cut.
The machines concerned cut the strand of tobacco in such a way as to obtain tobacco portions which are underweight compared to a preset weight required for subsequent packaging.
At the sides of the primary drum, in an underlying zone, there are two secondary drums, which are identical to the primary drum and into whose receptacles the primary drum itself releases the single portions after weighing them.
More precisely, the primary drum is moved reciprocatingly so as to release a portion to one of the secondary drums and then to the other alternately, thereby increasing the productivity of the machine.
Since the portion that is dropped into the secondary drum is underweight, the secondary drum itself is made to face an auxiliary compensating duct which adds tobacco to each portion in order to reach the required target weight.
The auxiliary duct leads out of the hopper, from which it draws the tobacco needed for compensation, and is made up of a succession of channels and rollers for transporting the tobacco.
The channels and rollers are functionally connected to the scale of the primary drum in order to convey to the secondary drum the quantity of tobacco to be added in order to reach the target weight.
Weight compensation is performed by allowing the tobacco to drop by gravity from an upper position, that is, from the outlet of the auxiliary duct, to a lower position, that is, the receptacle of the secondary drum.
Under each of the secondary drums there is a conveyor belt which mounts a succession of buckets moved by the conveyor itself.
The buckets are positioned one after the other under the secondary drums which, as they turn, allow each portion of tobacco to drop into a respective bucket.
By way of example, patent publications CA1220107 and EP1992924 describe machines for forming and individually transporting tobacco portions of the type described above.
The productivity of the prior art devices described above is low because the tobacco is gravity fed numerous times along its transportation path. Indeed, gravity feed constitutes a physical limitation which makes it impossible to increase production speed.
Moreover, the system by which the weight of the tobacco portions is checked and compensated is unsatisfactory because weight checking and compensation cannot be carried out until after the tobacco portions have been cut from the continuous strand of loose tobacco.